The present invention relates to transmission systems using optical fibers, and more particularly, to an optical communication system including a plurality of optical amplifiers for transmitting optical soliton pulses.
An optical fiber communication technique, ultra-long distance systems are now developing on the babe of a high-pace progress of the optical amplification technique, so that an optical transmission system over the Pacific Ocean can be realized without using any regenerative repeater. However,communication speed and capacity of the conventional optical communication system are limited since the transmission performance is degraded due to the wavelength dispersion characteristic and the nonlinear optical effect,as the communication speed becomes high. An optical soliton communication system has been closed up as a system capable of eliminating the limitation caused by the wavelength dispersion characteristic and the nonlinear optical effect. This optical soliton communication system is positively using the wavelength dispersion characteristic and the nonlinear optical effect,which are factors of the degradation of the transmission characteristic. The optical soliton pulses can be transmitted without deformation of the pulse shapes since the expansion of the pulse width due to the wavelength dispersion of the fiber is balanced with the reduction of the pulse width due to the nonlinear effect of the fiber. The optical communication system using such optical soliton pulses are now researched to realize a useful communication system because of many advantages,such as capability of large capacity,easiness of multiplexing and no additional deterioration by the nonlinearity of the optical fiber in comparison with a conventional optical communication system. In order to realize an ideal operation of the optical soliton pulses, it is essential that the optical fiber has no loss and the wavelength dispersion D and the soliton peak power P.sub.sol meet with the following equation (See a literature: L. F. Mollenauer et al., Journal of Lightwave Technology, Vol. 9, pp 194-197, 1991): ##EQU1##
In this Equation (1), .lambda. is the wavelength of the optical signal; A.sub.eff the effective area of an optical fiber; c the light speed; n.sub.2 the nonlinear coefficient of the optical fiber; .tau. the full width of the half maximum of the optical soliton pulse.
An actual optical fiber has the loss. Therefore, even if the peak power of the optical pulse and the wavelength dispersion are mutually balanced at the input end of the optical fiber, the effect of the wavelength dispersion increases as the optical pulse transits through the optical fiber, because the decrease of the peak power causes and the pulse broadening and disables the optical soliton pulse operation.
To compensate for this deterioration, a system called as a dynamic soliton transmission was proposed for a long-distance pulse optical soliton pulse transmission system (See: N. Nakazawa et al, IEEE Journal of Quantum Electronics, Vol. 26 pp 2095-2102, 1990),in which optical power loss is compensated for by the optical amplifiers and the peak power of the optical pulses at the input end of each optical fiber is reset to a value a little more than the power defined by each Equation (1).
In the dynamic soliton transmission system, the optical pulse width is compressed due to the effect of the nonlinearity of the optical fiber caused by the high peak power of the optical pulses at the initial portion of the optical transmission. However, at the end portion of the optical transmission where the power of the optical pulses are attenuated by the loss of the optical fibers, the pulse width is broadened by the effect of the wavelength dispersion of the optical fiber. To compensate for this pulse broadening, an optical amplifier can be inserted at a position where the pulse width is returned to the initial value so that the optical soliton pulse operation can be maintained in the optical transmission system. In this case,it is required that a section-average power meets with the condition defined in Equation (1). In FIG. 6, the relationship between the optical pulse width and the optical pulse peak power is described.
The wavelength dispersion in Equation (1) is defined as an average value in a transmission section, and the section length Z.sub.o is shorter than a length Z.sub.o defined by the following equation: ##EQU2##
In this Equation (2), D is an average value of the wavelength dispersion of the optical fiber of the transmission section; .lambda. the wavelength of the optical signal; c the light speed; .tau. the full width of the half maximum of an optical soliton pulse. A length Z.sub.o defined by Equation (2) is usually called as "soliton period". If the average value of the wavelength dispersion D of the optical fiber of the transmission section meets with the condition defined in Equation (1) and the section length Z.sub.o is sufficiently shorter than the soliton period Z.sub.o, then the optical soliton pulse transmission can be performed.
In the dynamic soliton pulse transmission, the peak power of the optical soliton pulse is controlled so as to mutually equalize the pulse widths at the input end and the output end of the optical fiber, which are connected between adjacent two of the optical amplifiers. In this case, the average value of the wavelength dispersion should be maintained for each span of the optical fiber.
However,from the viewpoint of manufacturing deviation of the optical fiber, it is impossible to maintain the average value of the wavelength dispersion for each span of 30 Km to 50 Km in an ultra-long distance optical communication system, such as the Trans-Pacific System.